I did a few builds directly on the build platform and they worked very well except for one issue. The area in direct contact with the base has a ridge that varies in thickness relative to the desired dimension. The ridge is thinner closer to the center of the build platform and bigger farther away. This seamed curious until I considered how the laser was scanning the area. In the center it hits nearly perpendicular to the build platform. At the edges it hits at an angle. What is happening is for the first 1 to 2 mm of build the laser is scattering off the build platform and curing additional material. When the laser hits vertically most of the scattered energy is in line with the laser. When it hits at an angle more or the energy is off axis and outside the part build area. The result is the larger the angle the laser hits the platform the larger the cured ridge is.
So there are three ways to improve this. One is to have the laser hit square on over the whole surface. This would require a redesign of the optical system and a very expensive curved front surface mirror or a large lens neither very practical. The second solution is to absorb the light in the build platform by making it black. So the reflective energy can be decreased. The bare aluminum reflects about 85% of the light that hits it. A black anodized surface should drop the reflected energy to about 10% to 15% and drastically reduce the over cure. The third possibility is to modify the scan profiles for the first 2mm of layers to take the height above the build platform and the laser scan angle into effect. Essentially under scanning slightly in the direction of scatter and counting on the scatter to cure the needed additional material. This would take some time to characterize how much the under scan should be and would even need to to be repeated for each new material. The best accuracy could be achieved by combining the two ideas.
The black build surface would be a quick an inexpensive change that would reduce the amount of post processing needed to eliminate the direct build ridge and allow for more material and time saving direct builds without the need for supports and base.